Android JNI

Definition: JNI is a framework that allows Java code to interact with native code (code written in other languages like C/C++). It's a crucial component for Android development when you need to use native libraries or optimize performance-critical sections of your app.

Purpose:

• Accessing Native Libraries: JNI lets you call functions from native libraries (like PJSIP, SQLite, etc.) directly from your Java/Kotlin code.
• Performance: Native code can be more efficient than Java code for certain tasks, especially when dealing with complex algorithms or low-level operations.
• Hardware Access: You can use JNI to interact directly with hardware components or system APIs.
• Code Reuse: If you have existing C/C++ code, you can reuse it in your Android app via JNI.
• Change the code: You can use JNI to change the code of the library.
• Accessing native libraries: You can use JNI to access native libraries.

Key Concepts in JNI Programming

Native Methods:

• Declaration: In your Java/Kotlin code, you declare native methods using the native keyword. These methods don't have a body in the Java/Kotlin code; their implementation is provided in your native C/C++ code.

   1:      class MyNativeClass {

   2:          external fun myNativeFunction(input: String): String

   3:          // ... other methods

   4:      }

Implementation::

• In your C/C++ code, you'll implement the corresponding native function. The function name follows a specific naming convention based on the Java method name.

   1:  extern "C" jstring 

   2:  Java_com_example_ndk_MainActivity_stringFromJNI( JNIEnv* env, jobject thiz)

   3:  {

   4:      return (*env)->NewStringUTF(env, "Hello from JNI !");

   5:  }

JNIEnv (JNI Environment):

• Interface: JNIEnv is a pointer to a structure that represents the JNI environment. It provides a set of functions for interacting with the Java Virtual Machine (JVM).
• Accessing Java Objects: You use JNIEnv to create, access, and manipulate Java objects from your native code.
• Calling Java Methods: You can use JNIEnv to call Java methods from your native code.
• Exception Handling: JNIEnv allows you to handle exceptions thrown by Java code.
• JNIEnv: JNIEnv is a pointer to a structure that represents the JNI environment.

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